The present invention is directed to polymers that when used with a suitable photoacid generator (PAG), form a positive working photoresist which is particularly suitable for use at extremely short wavelengths such as 193 nm.
Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then heated to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The coated surface of the substrate is next masked and subjected to exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this masked exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
There are two types of photoresist compositions, negative-working and positive-working. When negative-working photoresist compositions are exposed to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution while the unexposed areas of the photoresist coating remain soluble to such a solution. Thus, treatment of an exposed negative-working resist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating, thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited.
On the other hand, when positive-working photoresist compositions are exposed to radiation, those areas of the photoresist composition exposed to the radiation become more soluble to the developer solution while those areas not exposed remain insoluble to the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
After development, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gas and the like. The etchant solution or plasma gases etch that portion of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material, which corresponds to the photomask used for the exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining photoresist layer, after the development step and before the etching step, to increase its adhesion to the underlying substrate and its resistance to etching solutions.
Photoresist resolution is defined as the smallest feature which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In most manufacturing applications today, resist resolution on the order of less than one micron is necessary. In order to provide such so called sub-micron resolution photolithography processes have used increasingly shorter wavelengths of light. Photolithography processes have progressed from visible light, to ultraviolet (I-line) to deep ultraviolet (248 nm) to the wavelength of 193 nm, which can be generated by argon/fluoride lasers. In addition, it is almost always desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate.
As semiconductor manufacturing processes seek higher and higher resolutions many previously useable photoresist compositions become unworkable. Many photoresist compositions, such as novolacs and alkoxy styrenes, which contain a phenyl group, are simply not transparent at wavelengths below 240 nm and others do not dissolve sufficiently cleanly to provide the requite resolution. The polymer formulation of the present invention is sufficiently transparent to wavelengths above 180 nm so as to be useable in photoresist compositions at these wavelengths. In addition, the mechanism for photoacid catalyzed cleavage results in the efficient formation of highly soluble, small molecule fragments which will give higher contrast images than the conventional resin approach which only gives rise to a solubility change in the polymer. The present polymer, which contains the active acetal protected .alpha.-hydroxy anhydride group, when used with a suitable photoacid generator, will decompose into a series of small molecular weight, water-soluble fragments, as cleavage of the polymer will occur at both the acetal and anhydride sites. The products from this photoacid cleavage are significantly more polar, which creates a large solubility difference between polymer and product, resulting in high contrast and high resolution.